Much of the recent focus of melanoma targeted therapy has been on the MAPK pathway, which is aberrantly activated in approximately 90% of melanoma tumors (over half of which express BRAFV600E). Current targeted therapies such as Vemurafenib (BRAFV600E inhibitor), or a combination therapy of BRAF and MEK inhibitors produce a profound initial response in a majority of BRAFV600E expressing tumors. However, these responses are often short-lived and resistance typically develops within months. Genomic analysis has failed to identify resistance mechanisms in >40% of human cutaneous melanoma tumors. Identifying novel BRAFi-resistance pathways will yield new drug targets and therapeutic approaches to treat melanoma. Sox10 downregulation promotes BRAFi resistance. My preliminary data suggests that cell line models of acquired Vemurafenib resistance with low Sox10 activate the RhoA pathway. In addition to the role of RhoA in regulating the cytoskeleton, RhoA can also regulate gene expression by activating multiple transcriptional co-activators (such as MRTF- A and YAP1). My preliminary data demonstrates that MRTF-A and YAP1 are also activated in Sox10Low BRAFi-resistant cells. The objective of this proposal is to identify the downstream mechanisms by which Sox10 loss promotes BRAFi resistance. This is important, since Sox10 loss is a major Vemurafenib resistance mechanism, but we lack approaches to effectively target these cells. Sox10 loss may also promote metastasis, and immune-evasion, so finding ways to target Sox10Low cells could be beneficial for other aspects of melanoma therapy. Using an integrated approach, work in this proposal will combine bioinformatics, molecular and cellular biology, pharmacology, and pre-clinical in vivo xenograft models I will: 1) Identify the mechanisms by which Sox10 loss promotes BRAFi resistance and 2) Test combination drug therapies targeting RhoA-regulated gene transcription and the MAPK pathway using in vivo PDX models. The long-term goal of this work is to develop effective therapies for the improved treatment of melanoma. Successful completion of this proposal will provide pre-clinical rationale for designing combination therapies targeting the MAPK pathway and RhoA-regulated gene transcription.